1. Field of the Invention
The present invention relates to a combustion chamber structure of a gas engine, in which a gas fuel is supplied to a pre-combustion chamber with the communication between the main combustion chamber and the pre-combustion chamber cut off, followed by a throat hole being opened to introduce air into the pre-combustion chamber to mix the air and the gas fuel for ignition and combustion.
2. Description of the Prior Art
Gas engines with a lean-fuel combustion scheme have been developed to realize reduced NOx emissions and high efficiency. Such engines include those having a gas mixer in an intake manifold and burning the fuel in the Otto cycle and those having a pre-combustion chamber into which a gas fuel is supplied for combustion.
For example, Japanese Patent Laid-Open No. 310550/1995 filed by this applicant discloses such a gas engine.
The gas engine disclosed in the Japanese Patent Laid-Open No. 310550/1995 introduces a gas fuel such as natural gas into a pre-combustion chamber, compresses only the intake air in the main combustion chamber to a high compression ratio, detects the cylinder inner pressure in the pre-combustion chamber with a sensor such as piezoelectric element, operates based on this information a fuel supply valve to control the fuel supply to an appropriate amount according to the load and revolution, with the air in the main combustion chamber heated to a high temperature, opens the throat hole valve to allow the highly compressed air to flow into the pre-combustion chamber so that the gas fuel in the pre-combustion chamber can be mixed at once with the highly compressed air and ignited and burned in a short period of time. With this process, an excessively rich air-fuel mixture is burned in the pre-combustion chamber to suppress the production of NOx and flames are blown out quickly from the pre-combustion chamber into the main combustion chamber to effect the secondary combustion under as uniform an air-fuel mixture condition as possible and complete it in a short period, thereby reducing the production of NOx and HC and improving thermal efficiency.
In the above gas engine, it is necessary to supply a gas fuel into the pre-combustion chamber when the pressure in the pre-combustion chamber is low with the throat hole valve closed and, in the latter half of the compression stroke, to open the throat hole valve to supply high-temperature, high-pressure air into the pre-combustion chamber to mix the air and the gas fuel rapidly and cause self-ignition of the mixture so that the burning mixture blows out into the main combustion chamber by the rising pressure caused by combustion in the pre-combustion chamber thus completing the lean-fuel combustion in a short period of time.
With the above gas engine, however, because the throat hole valve is opened in the latter half of the compression stroke, the actual compression ratio in the main combustion chamber becomes high increasing the compression work done by the piston, which is consumed in friction and other losses, so that the net effective work decreases, degrading the thermal efficiency.
To solve the above problem with the conventional gas engines, it is conceivable to open the throat hole valve early in the compression stroke to introduce the compressed air from the main combustion chamber to the pre-combustion chamber. This however results in a phenomenon in which the gas fuel in the pre-combustion chamber disperses into the main combustion chamber entering into crevice regions of the piston, which is considered detrimental to improving the combustion performance. From the standpoint of suppressing the production of NOx, lean fuel is preferable for the gas engines and, to effect the lean fuel combustion using a gas fuel, reliable ignition and rapid combustion of lean air-fuel mixture are essential.